Embodiments of the present invention relate to a seal assembly for a valve stem. Such seal assembly is used in particular to prevent leaks of process fluid which is toxic or harmful to the environment.
A seal assembly of this type is known in the art. For example, document US 2004/0227302 shows a dynamic sealing arrangement for preventing leakage of a fluid along a shaft. Such arrangement comprises multiple seals which hold pressurized barrier fluid. According to one aspect, at least three seals are arranged along the shaft with pressurized barrier fluid between the first and second seals. A collection area is provided between the second and third seals to collect any leakage that might occur. Collected leakage, if any, is ported to an output port. According to another aspect, a unitary seal block member is provided that supports the dynamic sealing arrangement for the shaft and defines a piston chamber and a drilled passageway connecting the piston chamber and barrier sealant pressurization chamber between two seals.
Indeed, in such sealing arrangement the piston has a process surface, exposed to the process fluid, and an internal surface, which is in contact with the barrier fluid inside the chamber. The piston also comprises a stem which is exposed to the external environment. As a consequence of this arrangement, the pressure from the process fluid is sufficient to pressurize the barrier fluid to a pressure higher than the process pressure, since it acts on a bigger surface (the process surface is equal to the sum of the internal surface plus the cross section of the stem).